February 2, 2000 - Common
milkweed (Asclepias syriaca) is one of the few native prairie plants that can
survive the annual disturbances that occur during production of corn and soybeans, thus
this species is found both in row crop fields and in less disturbed habitats. Common
milkweed plays an important role in the life cyle of the monarch butterfly in that adult
monarchs lay their eggs exclusively on species in the milkweed family and the larvae feed
only on these plants. Although there are 17 milkweed species found in Iowa, common
milkweed is the most abundant and believed to be the most important food source for
monarch larvae in Iowa and the northern corn belt. This past year there has been
increased interest in common milkweed and monarchs due to potential impacts of biotech crops on populations of both
organisms. To better evaluate these concerns, an increased understanding of the
distribution of common milkweed in the midwest is needed.

We conducted a state wide survey to determine the occurrence
of common milkweed in different habitats. Forty 25 square mile cells were randomly
selected across the state. Ten 1.2 acre sites were selected within each of these
cells to determine the presence of common milkweed. All of the sites were located
adjacent to a road and consisted of an area of roadside vegetation and an area in corn or
soybeans (occasionally a site would be maintained in pasture or some other crop).

Common milkweed was most frequently found in roadsides,
whereas pastures had the lowest infestation rate (Table 1). Approximately half of
the corn and soybean fields were infested with milkweed with an average of 2.8 milkweed
patches per acre. None of the fields surveyed (179 corn fields, 153 soybean fields)
had a milkweed infestation that would be expected to impact crop yields. Fields in
the Conservation Reserve Program (CRP) had the highest percentage of land infested with
common milkweed.

Table 1. Distribution of common milkweed in
Iowa, 1999.

Vegetation type

Percent of sites
infested
with common milkweed

Number of common
milkweed patches per acre

Percent of area
infested with common milkweed

Roadside

71

19.4

1.0

Corn

46

2.8

0.3

Soybean

57

2.8

0.2

Pasture

28

2.8

0.1

CRP

67

3.6

2.1

Land maintained in CRP most closely resembles the native
habitat of common milkweed, thus the high percentage of milkweed in this land is not
surprising. Much of the CRP land is found in the southern half of Iowa, and milkweed
found in CRP probably is an important resource for monarchs in this part of the state.
There are approximately 800,000 acres of roadside in the state, and the uniform
distribution of this habitat may increase its importance for monarchs. While common
milkweed was not found at high densities in corn or soybeans, it was present in over 50%
of the surveyed fields. The dominance of corn and soybean in certain parts of Iowa
results in this being the primary habitat for milkweed in many parts of the state.

Corn pollen is relatively heavy and does not move long
distances with wind, thus any risk to monarch larvae from Bt pollen is likely to occur
with milkweed growing in or within a short distance (<10ft) of Bt corn fields.
Most corn fields share a border with a roadside, thus common milkweed in roadsides could
accumulate significant quantities of pollen on their leaves. We conducted a second
smaller survey to evaluate whether milkweed was evenly distributed across the width of the
roadside. Significantly less common milkweed was found in the area of the roadside
immediately adjacent to the cultivated field. This could reduce the risk to monarchs
using milkweeds found in this habitat.

Position

% of roadside
infested
with common milkweed

0-10 ft from field

0.9 b

10-20 ft from field

1.6 a

20-30 ft from field

1.9 a

Research conducted the past year has shown that pollen from
most Bt hybrids is only moderately toxic to monarchlarvae. However,
there continues to be debate on the risk of Bt corn to monarch populations due to an
incomplete understanding of monarch behavior and the impact of Bt pollen on monarch
fitness. One important aspect of monarch behavior that remains unknown is whether
the butterfly preferentially lay eggs on milkweed plants growing in different habitats.
If monarchs avoid milkweed growing in or near corn fields, then it is unlikely that
pollen from Bt corn poses much of a threat to this species. The EPA is requiring
manufacturers of Bt corn to generate additional data on numerous aspects of this issue, so
our understanding of the interaction between these two organisms will continue to
increase.

Common chemical and
trade names are used in this publication. The use of trade names is for clarity by the
reader. Inclusion of a trade name does not imply endorsement of that particular brand of
herbicide and exclusion does not imply nonapproval.